The present invention can be best understood and appreciated by undertaking a brief review of the crude oil distillation process, and most particularly, the critical role coker charge heaters play within that process.
In its unrefined state, crude oil is of little use. In essence, crude oil (a.k.a. hydrocarbon) is a complex chemical compound consisting of numerous elements and impurities. Such impurities can include, but are not limited to sulfur, oxygen, nitrogen and various metals that must be removed during the refining process.
Refining is the separation and reformation of a complex chemical compound into desired hydrocarbon products. Such product separation is possible as each of the hundreds of hydrocarbons comprising crude oil possess an individual boiling point. During refining, or distillation, crude oil feedstock temperature is raised to a point where boiling begins (a.k.a. "initial boiling point," or "IBP") and continues as the temperature is increased. As the boiling temperature increases, the butane and lighter fraction of crude oil are first distilled. Such distillation begins at IBP and terminates slightly below 100.degree. F. The fractions boiling through this range are represented and referred to as the "butanes and lighter cut."
The next fraction, or cut, begins slightly under 100.degree. F. and terminates at approximately 220.degree. F. This fraction is represented and referred to as straight run gasoline. Then, beginning at 220.degree. F. and continuing to about 320.degree. F. the Naphtha cut occurs, and is followed by the kerosene and gas/oil cuts, occurring between 320.degree. F. and 400.degree. F., and 450.degree. F. to 800.degree. F., respectfully. A term-of-art "residue cut" includes everything boiling above 800.degree. F.
The residue cut possesses comparatively large volumes of heavy materials and two fundamental processes are employed to convert appreciable amounts of such residuals to lighter materials--thermal cracking and delayed coking. While thermal-cracking may be properly considered "the use of heat to split heavy hydrocarbon into its lighter constituent components," delayed coking should be considered "severe thermal cracking" and occurs within a coke drum after a coker feedstock has been heated in an apparatus referred to as a coking heater, or "delayed coker charge heater." An improved delayed coker charge heater and process serve as the focus of the instant invention.
In the present art, fresh feed is preheated by preheat exchangers exclusively, prior to introducing such feed to combination tower for processing. Fresh feed of a cold variety is first introduced to the refining process from tankage, while hot feed is introduced to the refining process from a vacuum distillation unit, or units. As preheat exchangers foul from use, their operating efficiency diminishes over time. Less efficient preheat exchangers, in turn, occasion temperature variances in fresh feed prior to its introduction to a combination tower. Coke drum overhead temperatures and flow rates also vary, as do heavy gas only (HGO) quench rates, both contributing to the instability of residuals temperature at the bottom of a combination tower. Especially large temperature swings occur when switching from a hot coke drum to a cold coke drum. Consequently, a combination tower's bottom temperature varies greatly based upon input from the afore stated sources.
The utilization of coker pre-heaters are well known in the art. As intended, the purpose of a coker pre-heater was to preheat process fluid, or coker feedstock, when the unit was designed without an effluent exchanger train or with a minimally configured feed/effluent exchanger train. In such instances, temperature control and firing rates are based on pre-heater outlet temperature. The present art clearly lacks innovative consideration to maintain, stabilize, and control combination tower "bottoms" temperature. In providing for such maintenance, stabilization and control, the present invention introduces a novel means by which "temperature stabilized" combination tower bottoms can be introduced to a coker charge heaters, and thus, maintain and stabilize the firing rates of such heaters.
Hence, given the deficiencies of the present art and improvements afforded by the instant invention, what is needed is an improved system and method to effect and control temperature of coker process fluid, in advance of introducing such fluid to a coker charge heater.